instrument such as the present, where practical construction may not have the correctness of its theoretical one, it would appear that the more accurate way of dividing the scale will be to do so from the indications of a standard barometer of common construction.
In the case of the cup float there will be a tendency to move the tube from the perpendicular, in consequence of the lower portion of the tube being so much lighter than the upper. This tendency, however, is easily prevented by lowering the centre of gravity of the tube, for which purpose two rods attached to the tube may descend outside the cistern to a convenient distance below it, to support a disc on which a weight may be placed. This weight will if sufficiently heavy bring the centre of gravity of the tube completely below the cup float. Indeed, if this float be sufficiently large to buoy up the tube, the balance part of the apparatus may be laid aside altogether, and the weight below the cistern, acting on the principle of ballast, will preserve the tube in the perpendicular. In order to keep the tube steady it would be necessary to have guides on either side of the tube—the pencil can be attached to any part of it, and made to bear directly on the register roller, which, as the tube moves perpendicularly, may be made in the manner described in the original plan, and in order to admit of the indications being magnified, the upper part of the float can be contracted.
Respecting form No. 2, (Fig. 1st, plate 15, Journal No. 14) the imperfection Mr. Taylor has pointed out as to the impossibility of knowing whether "air does not exist in the quicksilver between e and c," is certainly a very serious one and indeed applies not only to the part E C but to the entire tube, since the possessor of such an instrument would not, as in the glass tube, have the advantage of ocular demonstration as to the presence of air; but would be left in doubt on this essential point. Presuming that the objection to iron will be got over by stating it as above mentioned, the following arrangement appears to remove the imperfection under consideration. Let the end C of the metallic tube rise into the middle of a cistern, after the manner that the glass tube of the Englefield barometer descends in the iron cistern—so that in what position soever the instrument be held the termination of the tube will always be immersed in mercury. On the top of this cistern let there be a stop-cock from which rises the glass short limb—lastly, let a screw enter one side of the cistern, which, when screwed home, will occupy a considerable portion of its area. Suppose now the entire long limb filled with mercury, the stop-cock K turned to prevent its return, also the cistern (the screw being out as far as possible) filled with mercury. Let now the glass limb be attached, and the cistern screw screwed home. Mercury will now stand in the glass tube at a given height—which ought to be engraved on the instrument.
